A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Accurate control of the relative position and/or orientation of various elements of the lithography apparatus is desirable to ensure high performance (e.g. accurate overlay). Vibrations can interfere with such control by reducing the accuracy of the measurements and/or by causing undesirable movement of the body to be controlled. Encoder systems may be used for performing the position and/or orientation measurement. A sensor part on one body may be configured to detect radiation reflected from a pattern or grid on another body (which may be referred to as a reference part). Interferometry may be used. Vibrations of the sensor part or the reflective body can reduce the accuracy of the measurements.
A measurement system (e.g. encoder system) may be used for measuring the position and/or orientation of the substrate, of a pattern formed on the substrate, or of the substrate table relative to a reference frame. The measurement system may comprise an alignment sensor or a level sensor or both. In this context the reference frame is sometimes referred to as a “metroframe”. A plate comprising a grid (sometimes referred to as a “gridplate”) may be attached to the metroframe with the sensor part being attached to the substrate table or vice versa. For reasons of productivity it may be desirable to use substrates that are larger than the substrates that have been most commonly used previously, for example substrates having diameters of 450 mm or more, rather than 300 mm or less. Such substrates require substrate tables, metroframes and gridplates that are larger laterally (i.e. in directions parallel to the plane of the substrate). In order to maintain sufficient stiffness in the laterally larger gridplate, the gridplate may need to be made thicker. However, the amount of space available in the thickness direction (perpendicular to the plane of the substrate) may be limited. The reference frame may therefore have to be made thinner to accommodate the thicker gridplate.
Increasing the lateral size of the reference frame and decreasing the thickness of the reference frame will tend to reduce the frequencies of the natural internal modes of vibration or resonance (also referred to as the “eigenfrequencies”) of the reference frame. Lower eigenfrequencies may favor vibrations that are more problematic for measurement systems that interact with the reference frame. It has been proposed previously to use a plurality of smaller reference frames rather than a single reference frame to make internal eigenfrequencies higher. However, smaller reference frames are lighter and more easily accelerated, and therefore displaced, which may also lead to errors in position control.